1. Field of the Invention
The present invention relates to a mold for press-molding glass optical elements and a molding method using the mold, and more particularly to a mold for press-molding glass optical elements with high accuracy without the need of any further process such as a polishing process after the molding, and a molding method using the same.
2. Description of the Prior Art
Recently, a desire has arisen for cameras for video tape recorders and compact disc players which are small-sized, lightweight and low in cost, and therefore simplification of the lens system has been required. Since the conventional spherical lens is limited in simplification, an aspherical lens is required. However, the manufacturing of the aspherical lens by use of the conventional polishing method has a problem in mass-productivity.
For this reason, a tendency to manufacture the aspheric lens by a direct press-molding has recently been promoted.
One technical factor important in molding an aspheric lens with high accuracy by direct pressing is the material used for the mold. Namely, it is important in the mold for directly press-molding the glass lens with high accuracy that (1) the material of the mold surface does not react on the glass at a high temperature (or the glass does not adhere to the mold surface), (2) the press surface of the mold is hard enough not to be damaged by a scratch or the like, (3) the mold is not deformed at a high temperature, (4) the mold is strong enough even at a high temperature, and (5) the mold is superior in resistance to heat shock.
The mold for directly press-molding glass optical elements has until today been reported to use silicon carbide (SiC), silicon nitride (Si.sub.3 N.sub.4), titanium nitride (TiN), titanium carbide (TiC), vitreous carbon, tungsten carbide (WC), or nickel group alloy.
However, since SiC, Si.sub.3 N.sub.4, and TiC are each of an extremely high hardness, it is very difficult to form these materials each into an aspherical shape with high accuracy. Moreover, these materials and WC are all to be sintered, and thus a third component is added as a sintering agent. Since the third component is easy to react on the glass, it makes it impossible to mold the glass optical element with high accuracy. Also, the press-molding using the aforesaid materials, vitreous carbon and titanium nitride, is defective in that the press surface of the mold is oxidized and thus deteriorates unless the concentration of O.sub.2 is controlled to be low. For the nickel group alloy, the grain growth is generated under pressing at about 500.degree. C., so that the press surface of the mold becomes rough. Futhermore, the glass, especially lead oxide series glass, is apt to be attached to the nickel group alloy. Hence, the nickel group alloy is not suitable for molding the glass optical element with high accuracy.